South Esk – Great Lake
Water Management Review
Scientific Report on
Lake Augusta
August 2003
Prepared by
Hydro Tasmania
Table of Contents
1.
2.
3.
4.
Assessment of Issues and Status............................................................ 1
Formulation of Study Objectives............................................................ 10
Data Collection and Analysis ................................................................. 11
Environmental Management Options for Lake Augusta......................... 26
Scientific Report on Lake Augusta
August 2003
LAKE AUGUSTA
1. PRELIMINARY REVIEW
Geomorphology
During the Environmental Review component of the South Esk – Great Lake
catchment Water Management Review, it was noted that wind blown sand dunes
(lunettes) of potential geoheritage significance border the eastern shore of the
original Lake Augusta (see Figure 1). A brief study was carried out on the area by
the Parks and Wildlife Service in 1994 (Bradbury, 1994). This report suggested that
the frequent elevation of water level in Lake Augusta has resulted in erosion of the
dunes, and that these may be degraded in part as a result of the raising of Lake
Augusta. While the lunettes are not listed in the World Heritage Area, the lake and
the land vested in Hydro Tasmania is surrounded by the World Heritage Area. The
primary stakeholders affected by this issue are the Parks and Wildlife Service and
people concerned with geoheritage issues.
The lunettes identified in the area surrounding Lake Augusta (referred to as the
Nineteen Lagoons area), are derived from doleritic sand and are thought to have
formed between 4,000-5,000 years ago. These lunettes are considered outstanding
earth heritage features (Bradbury, 1994).
Bradbury (1994) assessed degradation of the landforms throughout the Nineteen
Lagoons area and concluded that the integrity of many of the aeolian landforms in
this region had been compromised, and some features had been completely
destroyed as a result of human activities. The largest aeolian landforms in the
Nineteen Lagoons area (dunes of up to 5 - 6 m in height) occur on the eastern shore
of the natural Lake Augusta. Bradbury states that the formation of the present-day
Lake Augusta and the periodic raising of the lake level have resulted in degradation
of the lunettes through extensive lake-shore erosion. He listed activities such as
earthworks associated with electricity infrastructure and roads, access to the lake for
angling, physical trampling, fire, vegetation destruction from firewood collection for
campfires, grazing and burrowing by introduced animals as also having some impact
on the integrity of dunes.
The dunes are considered to be susceptible to wave attack as the soils are less than
5,000 years old and the fact that the alpine vegetation does not promote the
development of a strong protective root mat. Soils developed on the Lake Augusta
lunette and low lunettes around the Carter Lakes are uniform in texture with
undifferentiated profiles (Pemberton, 1986:30). These are generally yellowish-brown,
reddish-grey or greyish-brown. The soils of this lunette have been extensively
burrowed by wombats and rabbits. Black organic soils are common in poorly
drained locations at low elevations.
The raising of lake levels caused most vegetation and soils between the former
shoreline and the new level to be lost. Bradbury suggests that the development of
wave-cut scarps has allowed blowouts to be initiated. On the eastern shore of Lake
Augusta, Bradbury (1994) identified at least 6 active blowouts, originating at the
shoreline. At the time this study was carried out, some of these blowouts covered an
area in excess of 2 ha, while the dune front was estimated to have advanced
eastwards by at least 100 m.
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Scientific Report on Lake Augusta
August 2003
Figure 1: Area map of Lake Augusta showing general geomorphology
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Scientific Report on Lake Augusta
August 2003
Lake Augusta
James River
Outfall
One Tree
Beach
Short
Beach
Rocky
Beach
A
B
Long
Beach C
N
D
250
0
m
Legend
Sand lunette and lake shoreline ridges
Beach deposits
Cuspate spit
Low boulder plain
Nearshore silt and clay
Low quartzose sand ridges
Deflation corridor
Jurassic dolerite
Eroded foreshore
W ater
Rocky beach
Map prepared by
Hydro Tasmania
Environmental Services
Figure 2: Geomorphic Units in the area to the east of the original Lake Augusta (figure also
indicates location of cross-sectional profiles).
At the Augusta impoundment, Bradbury suggests that the intermittent raising of
the lake level has resulted in loss of vegetation as a result of waterlogging and that
the soil has then been removed by a combination of wind and wave action. He
suggests that this has resulted in the underlying morainal material being exposed to
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Scientific Report on Lake Augusta
August 2003
wind erosion, creating a deflation lag, which is at least partially developed across
the entire inundation area. In many areas of the Augusta impoundment, the
deflation appears to have been complete and the lag now fully armours the bed of
the impoundment.
A new lunette has formed just above the full supply level at the eastern end of the
Augusta spillway. This substantial dune, up to 4 m in height has formed since the
completion of the dam in 1953. This dune is well-vegetated and Bradbury (1994)
indicated that it is in better condition that any other aeolian landform in the
Nineteen Lagoons area. This dune gives some insight into the possible timeframe
for formation of other dunes in the area, and although this dune may not be
regarded as a geoheritage feature, it may be considered a cultural feature due to it
having formed as a result of human influence.
Hydro Tasmania currently manages the lake above a minimum level to prevent
sand blowing into the intake area. Bradbury (1994) suggests that the most effective
rehabilitation would be to return the sand currently stored in the littoral zone back
to the dune system. To do this, he suggests that there would need to be an
increase in the low lake level events, exposing the littoral zone to desiccation and
aeolian action. Sand fences may be necessary to trap sand remobilised in this
manner in areas of active blowouts.
In his report, Bradbury also suggested that rehabilitation of the degraded Augusta
dunes is likely to be impossible without Hydro Tasmania changing the operation of
Lake Augusta to increase the number of low lake levels. He also recommended
that a maximum lake level should be set for Lake Augusta and that the duration of
high lake levels should be minimised to prevent further degradation of the Augusta
dunes. No maximum lake level was suggested, however it would need to be below
the current full supply level. It was recognised that a reduced maximum lake level
may not present a long term management solution, but that increasing the capacity
of Liawenee Canal to more quickly drain the lake may allow the same potential
harvest of water resources without further serious impact on the Augusta dunes.
Hydrology
The original Lake Augusta is a relatively shallow body of freshwater estimated to
be about 2 m deep. It is a permanent overflow system fed mostly by the James
River. The fetch across the lake is approximately 2 km. Surface water is also
contributed by local rainfall and run-off. The lake waters exit through the James
River outfall. The James River continues to the Augusta Impoundment some 4km
to the east where it joins the River Ouse.
Construction of Augusta Dam across the Ouse River to divert water into the
Liawenee Canal produced a permanent body of water behind the dam (the Augusta
Impoundment) and altered the hydrology of the original Lake Augusta. During
high inflows, water is retained behind Augusta Dam causing Augusta
Impoundment to fill. As the impoundment fills, it backs up, flooding back into the
original Lake Augusta.
Hydrographic records kept by Hydro Tasmania since construction in 1953 show
that impoundment levels have fluctuated between a low of 1142.35 metres above
sea level and spills above full supply level at 1150.62 metres above sea level.
As expected, lake level is highly seasonal. High lake levels usually occur in winter
and early spring coincident with seasonally high rainfall and snowmelt in the upper
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Scientific Report on Lake Augusta
August 2003
catchments of the James River and Ouse River. This is also when westerly winds
are strongest. Conversely, lake levels fall in late summer when evaporation rates are
high and water continues to be transferred to Great Lake via Liawenee Canal. The
original lake remains relatively low (essentially within its natural boundaries) for
months at a time during the summer and drier parts of the year. Therefore it is
only during high lake levels (i.e. spill events) that the natural water level regime of
Lake Augusta is modified.
Cultural Heritage
Another issue that was raised during the Environmental Review was the potential
presence of sites of high Aboriginal Heritage significance in the area around Lake
Augusta, particularly along the eastern shoreline. It was noted that this might be
an issue in relation to the possible erosion of the eastern shoreline and sand dunes.
This issue affects the Aboriginal community of Tasmania.
In the case of Lake Augusta, cultural heritage issues are intrinsically related to
erosion issues. There are known Aboriginal sites in the Lake Augusta area, and
several surveys have been carried out previously. However, there has been no
assessment of the relative cultural importance of the area, or the susceptibility of
the sites to erosion as a result of water level management.
The shoreline of Lake Augusta has been partially surveyed for Aboriginal sites and
the state of existing knowledge has been summarised as part of a report by Smith
(1998), from the Tasmanian Aboriginal Land Council. This report indicates that a
significant proportion of the shoreline around Lake Augusta may contain
Aboriginal sites. The report classified all areas that could potentially contain
Aboriginal sites as areas of “high sensitivity”. The remainder of the shoreline of
Lake Augusta is lacking in information.
Other surveys that have been carried out around the shoreline of Lake Augusta
include:
!
Cosgrove (1984): this survey took in a small section of the north-west
shoreline of Lake Augusta, however no sites were recorded. One site was
recorded by Cosgrove at the mouth of the James River near Lake Augusta.
Aboriginal sites at that time were also known to be on the edge of Howes
Lagoon Bay and along Four Bays as the central southern side of the lake
(from Tasmanian Aboriginal Site Index records).
!
Du Cros (1992): Lake Augusta was surveyed as part of a report on the
distribution of Aboriginal sites affected by erosion in the World Heritage
Area - primarily the track across the dry middle of the lake and the northern
and eastern margins. Sites were recorded along the north and north-eastern
shoreline, especially around Worcester Bay and around the mouth of the
River Ouse. The sites recorded were generally in already eroded areas as
these were the areas targeted and with the best visibility.
!
Tasmanian Aboriginal Land Council (1996a): During the Tana Trawna
project, approximately 3 km of the track running along the western side of
Lake Augusta was surveyed from the junction with Augusta spillway road to
Little Blue Lagoon. Aboriginal sites were identified from this survey.
Sections of this site were monitored following this and results showed that
there is ongoing damage to the site and the artefacts themselves from
vehicular traffic (Tasmanian Aboriginal Land Council 1996b; 1997).
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Scientific Report on Lake Augusta
August 2003
All Aboriginal artefacts are protected under the Aboriginal Relics Act 1975. Section
4-(1) of this Act it states:
“Except as otherwise provided in this Act, no person shall, otherwise in
accordance with the terms of a permit granted by the Minister on the
recommendation of the Director –
a) destroy, damage, deface, conceal, or otherwise interfere with a relic;
b) remove a relic from the place where it is found or abandoned.”
This Act applies to Aboriginal sites anywhere in Tasmania, regardless of land
tenure.
Threatened Species
Threatened species were not identified as an issue during the environmental review
process. This is because Paragalaxias julianus was not listed as a threatened species
until 2000. Following the listing of the species, the presence of Paragalaxias julianus
in Lake Augusta was noted as an issue in relation to how the regulation of lake
levels may affect the well being of this species and its habitat. The primary
stakeholder in relation to this issue is the Inland Fisheries Service.
Electrofishing surveys and surveys on habitat preference for this species are
required to better understand potential threats to the long-term survival of the
species that may arise from lake level management.
Trout Fishery
During the community consultation stage, one stakeholder raised a concern in
relation to the trout fishery (This is reported in the Community Consultation Report:
South Esk – Great Lake Water Management Review in Section 3.2). The stakeholder
was concerned that summer water levels in the Augusta Impoundment should
continue to be managed so that the lake remained a productive trout fishery. This
is an issue that potentially affects hundreds of trout anglers throughout the state.
Lake Augusta is generally referred to by anglers as two separate water bodies, Lake
Augusta and Augusta Impoundment. When the lake level is down the lake recedes
into the artificial impoundment behind Augusta Dam (Augusta Impoundment) and
the original water body (Lake Augusta). This is generally the status of the lake for
most of the fishing season (drier period of the year).
When lake level is low, Lake Augusta is essentially a natural lake and is the second
most popular trout fishing water in the western lakes (Sloane & French, 1991).
The lake and also the associated section of the James River (which is flooded
during high lake levels) supports a very large population of wild brown trout and a
significant number of wild rainbows (French, 1994; Sloane & French, 1991).
Anglers generally believe that the native macrophyte beds in the natural Lake
Augusta play an important role in the health of the trout fishery.
Augusta Impoundment is known to yield consistent bags of brown and rainbow
trout (0.5 – 1 kg) and occasionally bigger. The angling theory is that the lower the
storage the more concentrated the trout, resulting in better yields at lower lake
levels (Sloane & French, 1991).
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Scientific Report on Lake Augusta
August 2003
The fishing season for the Western Lakes is August to April inclusive. However,
as part of the Western Lakes Fisheries Management Plan, developed by the Inland
Fisheries Service, public opinion is being sought as to whether the season at
Nineteen Lagoons area should be shortened to coincide with the opening and
closing of the gate at Lake Augusta. The gate is generally shut during late autumn
and is usually opened in the first week of October, although this can vary
according to weather.
Fishery issues at Lake Augusta have not been examined as part of the Lake
Augusta Assessment as no major change to the current lake level management is
proposed. The Inland Fisheries Service has however developed a Fishery
Management Plan for the Western Lakes, which includes Lake Augusta and the
Nineteen Lagoons region. This process involved public consultation, including
surveys and stakeholder workshops.
Public Access and Amenity
There was one response during the community consultation stage (and a general
comment made later by another stakeholder) that related to public amenity and
access. The concern was that vehicular access to the Western Lakes is cut off
during parts of the fishing season as a result of high lake levels that cause spill over
the spillway. This is an issue that could potentially affect hundreds of trout anglers.
The other concern related to access across the four-wheel drive track to Pillans and
Julian Lakes, which crosses the dry section between the original Lake Augusta and
the Augusta Impoundment. When the lake rises, this track is impassable.
The fishing season for the Western Lakes extends from August to April. Concerns
regarding access specifically for fishing are not relevant outside the fishing season,
as it is outside the legal period. The Parks and Wildlife Service presently restrict
access to the Western Lakes area by closing a gate located on the road between
Augusta Dam and Augusta Spillway during winter. The gate is closed in late
autumn and is generally opened again in the first week in October. Reasons for
the Parks and Wildlife Service restricting access include the protection of the
Pillans – Julian Track and other 4WD tracks from undue degradation during wet
periods. The closure of the gate is also important for safety reasons, as the lake
spills frequently in winter. The lake is mostly very shallow and can rise quickly,
therefore unless the gate is closed in winter, there may be a danger of people
getting trapped on the other side of the spillway or being swept off if they attempt
to cross.
Access Across the Spillway
An assessment of water level and spill records for Lake Augusta was carried out
for the period 1990 to 2000 to determine how often during the fishing season the
lake spills, and how often during the fishing season it is at a level where the lake is
impassable. As the gate is closed by the Parks and Wildlife Service during the
wetter winter and spring periods for safety reasons and for the protection of tracks
and habitat, the main concern for this assessment has been the frequency with
which the lake spills outside the period for which the gate is normally closed (i.e.
October to April). These are the spill events that restrict access solely on the basis
of the lake spilling.
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Scientific Report on Lake Augusta
August 2003
In general, Lake Augusta is higher in the winter months and more likely to spill,
and lower in the drier summer months, with spills being relatively infrequent
during the ‘open gate’ period. Since January 1996, Lake Augusta has spilled on six
occasions between the periods of October to April. The dates and duration of the
spills are outlined in Table 1.
Year
Month
Dates
Duration
(days)
1996
Jan
Feb
Oct
Oct
Mar
Oct
27th – 30th
2nd
1st – 4th
2nd – 12th
4th – 7th
1st – 6th
4
1
4
11
4
6
1998
1999
2000
Average
discharge of
spill (cumec)
5.35
0.41
10.4
8.52
2.80
6.36
Table 1: Dates and durations of significant spills over the Lake Augusta spillway between
1996 and 2000
The information suggests that the lake is relatively unlikely to spill during the
majority of the ‘open gate’ period (October to April) and when it does spill during
this period, the duration of the spill is on average, only 6 days. In terms of the
overall picture, this is a relatively minor issue and is likely to inconvenience only a
small number of anglers. No additional investigation of this issue is required.
Access Across Lake Augusta
According to ‘Western Lakes Fishery Management Plan’ (IFS 2002), the Pillans –
Julian 4WD track is the most popular 4WD track in the Western Lakes. The main
problems with the track across Lake Augusta are poor routing and a lack of
maintenance. The track is presently closed by Parks And Wildlife Service during
the early season (when conditions are generally still wet) to protect it from undue
degradation, although wet conditions can occur throughout the year. This track
can become impassable as a result of rising water levels before the lake itself
reaches spill level, however if the lake is spilling, there is no access to this track (as
the spillway needs to be crossed in order to reach the start of the track).
However the constraint to the passibility of the Pillans – Julian track is not the
level of Lake Augusta itself, but the level of the James River, which follows its
original course across the dry lake bed. A general ‘rule of thumb’ is that when the
level of the James River reaches the iron supports on the footbridge which goes
across the river, then it is too deep for four-wheel-drives to cross (M. Cousins,
Parks And Wildlife Service, pers. comm.). This level in the river is obviously
reached prior to the lake itself flooding. The James River is likely to have risen
with high rainfalls in the catchment even under natural conditions (ie. pre-hydro
development).
Although the lake level management of Lake Augusta can not influence the
number of times the track is impassable as a result of the James River rising,
management of lake levels may influence the duration for which the track is
impassable.
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Scientific Report on Lake Augusta
August 2003
To determine what influence the operation of the lake has on how frequently and
for how long the lake is impassable, hydrological investigations, (probably
including some modelling) would need to be carried out. However, as the level of
concern that has been expressed regarding this issue was relatively low, no
additional investigations were undertaken as part of this study.
Exotic Species
Canadian Pondweed (Elodea Canadensis) is an introduced aquatic weed that is found
in Lake Augusta. It is a secondary prohibited aquatic weed introduced from North
America. It prefers warm, shallow, slow moving water and forms fast growing,
dense beds which out-compete native macrophytes. Elodea is known to clog canals
and intake streams to the extent where regular removal is required. It has a wideranging distribution throughout Tasmania. It is likely to out-compete native
macrophyte species, and is also believed to provide habitat for trout and possibly
Paragalaxias julianus and/or some species of macroinvertebrates, however this is not
supported by scientific evidence.
Canadian Pondweed was observed in Lake Augusta in 1998, washed up on the
windward shoreline (D. Blühdorn, Hydro Tasmania, pers. comm.). No surveys of
Canadian Pondweed in Lake Augusta have been undertaken, and so the
distribution and extent of the aquatic weed in Lake Augusta is not known.
However, it is likely that it is not a major environmental issue at present. The
primary concern regarding its presence in Lake Augusta is the risk of it spreading
into the Western Lakes. This is unlikely; as to do this it would have to migrate
against the flow. There is no Hydro Tasmania influence upstream of Lake
Augusta, so it is unlikely to have entered Lake Augusta via a hydro structure or
operation. The most likely pathway by which the weed has entered the lake is
regarded to be from boats and boat trailers transferring it from other water bodies.
This issue has not been further investigated as part of the Lake Augusta study.
Riparian Vegetation
Impacts on riparian vegetation around the shoreline of Lake Augusta resulting
from periodic inundation was recognised as a potential issue, however there is little
information on which to base this concern.
A wide variety of alpine plant species have been described for the Lake Augusta
area by Pharo (1990). The major elements of this community include shrubs and
graminoids. Other elements include cushion plants, herbs, grasses and a single tree
species, the pencil pine, Athrotaxis cupressoides. Pencil pines are thinly dispersed
along the shore of the Lake Augusta lunette.
A preliminary assessment of the riparian vegetation community was made to
determine if fluctuations in water level were having a detrimental impact on the
condition of this vegetation (Wild, 2001).
Alterations in water levels do not appear to be currently having a direct impact on
the vegetation of the shoreline. Large trees and shrubs affected by inundation of
root systems tend to show this impact by reduced growth, wilting, stunted root
systems, stem base swelling, premature leaf fall or yellowing of older leaves
(Crawley, 1986). Although no exhaustive investigations were undertaken, a brief
visual survey did not find evidence of any of these symptoms. This is not
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Scientific Report on Lake Augusta
August 2003
unexpected because it is probable that since the dam was built inundation tolerant
species have colonised the areas of frequent inundation. Therefore the plants that
are currently found there are likely to be well adapted to inundation by water.
Further evidence for the plants being well adapted is the distinct change in
vegetation at the high water mark. Most of the shoreline is dominated by a low
growing fen of inundation tolerant species characterised by a high number of
herbs, grass-like and cushion species. This community has a relatively high cover
of vegetation and species diversity. The vegetation cover provides good protection
for the soil against erosive forces such as frost heave, wind and wave action.
Above the fen, shrub communities that are not as tolerant of inundation dominate.
These communities have a lower vegetation cover than the fen community and
lower species diversity. The vegetation provides moderate protection against soil
erosion when the lower strata are intact, but it is very susceptible to disturbance
such as trampling.
Shrubs on the lunettes on the eastern side of the impoundment are being degraded
by mobile dune systems. Aerial photo interpretation indicates that these processes
have been occurring even before the dam was built, and the cause has been
investigated through the geomorphological assessment.
The vegetation of the lunettes is being inundated by sand and is also dying back
from soil erosion at the root zone. Few of the existing plants are able to tolerate
the mechanical disturbance of moving sand and once areas are bare additional
stress from factors such as frost heave, summer drought and exposure makes
seedling establishment very difficult. Any steps that may help to stabilise the dunes
would be beneficial to the vegetation.
2. FORMULATION OF STUDY OBJECTIVES
Seven issues relating to the management of Lake Augusta were raised or identified
through environmental review and stakeholder consultation. These are:
!
erosion of the lunettes on the eastern shore;
!
possible impacts from remediation of the lunettes on Aboriginal cultural
heritage;
!
environmental conditions for threatened species;
!
the quality of the trout fishery;
!
public access and amenity;
!
the presence of Elodea; and
!
impacts on riparian vegetation.
Following the review of existing information, a number of these have been
considered minor issues and were not further investigated. The most significant
issues are those relating to erosion of the lunettes, aboriginal heritage and the
native fish species that presently occur in the storage. Additional information was
required to clarify the facts relating to these issues in order for viable management
options to be developed.
The main focus of the investigation was therefore to collect more detailed
geomorphological information on Lake Augusta. The objective for this assessment
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Scientific Report on Lake Augusta
August 2003
was to determine whether there are areas that are susceptible to erosion, whether
erosion is in fact occurring, and if so, whether it is a result of the regulation of lake
levels in Lake Augusta or other processes.
An assessment of Aboriginal heritage is dependent on the findings of the
geomorphological assessment. It is known that there are Aboriginal sites in the
area, and therefore the affected areas will need to be surveyed if any interventive
management activities are proposed following the completion of the study. No
further survey work was therefore undertaken.
Surveys of population status and habitat requirements/preferences for Paragalaxias
julianus were also undertaken to enhance knowledge of factors that may influence
the long-term survival of this species.
3. DATA COLLECTION AND ANALYSIS
Geomorphic Survey
Separate independent consultants (Tim Stone and Dr Wayne Stephenson) were
commissioned by Hydro Tasmania to provide an assessment of geomorphological
issues at Lake Augusta.
The aims of the geomorphic assessment were to:
1. use morphological interpretation to identify landforms that may be
susceptible to erosion processes, and specifically related to lake level
management;
2. identify other possible processes that are contributing to erosion;
3. establish the geoheritage significance of these susceptible areas; and
4. propose potential options for management of issues or additional research.
Geomorphic Background
The Central Plateau is dominated by Jurassic dolerite (Pemberton, 1986:18). This
intrudes deformed Palaeozoic basement rocks and Permian-Triassic sediments of
the Parmeener Supergroup. The intrusion caused uplift of the Central Plateau
along northwest and northerly of the lake.
Pleistocene glacial and periglacial events have shaped the surficial geology and
geomorphology of the Central Plateau (Pemberton, 1986:23). Pleistocene icesheets once covered the western part of the Central Plateau reaching almost to
Lake Augusta. The closest recognisable glacial deposits to Lake Augusta are glacial
erratics and hummocky moraine. These features are developed west of Lake
Augusta. The origins of Lake Augusta itself are unclear. Banks (1973) suggested
that a recent northward tilting was responsible. However, the outline of the lake
suggests that it is simply a product of structural jointing in the underlying dolerite.
The Lake Augusta lunette is located on the eastern shore of the original lake
(Figure 1) and is one of a small number of large sand lunettes formed around lakes
on the Central Plateau. Banks (1973) attributed these features to a Mid-Holocene
Arid Period claiming that the sand was derived from the deflation of glacial
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Scientific Report on Lake Augusta
August 2003
outwash. The Lake Augusta lunette has been described previously by Pemberton
(1986) and Bradbury (1994). Pemberton (1986:25) believed that the lunette was
partly morainal in origin because of its location close to the eastern extent of the
Pleistocene ice cap. Bradbury (1994) argued that the lunette formed from a lakebed
source during an episode of major drought. He discounted a littoral sand source
during a lake full phase because this seemed insufficient to account for the volume
of sand in the lunette.
The proposed Mid-Holocene Arid Period is about 7,000 to 4,000 years ago (Pharo,
1990:13). However, the only known radiocarbon age relevant to the formation of
the Lake Augusta lunette is a date of c.3,700 years BP (Before Present) obtained
from an ‘organic palaeosol’ beneath a lunette at Lake Fergus located 10km south of
Lake Augusta (Cullen, 1995). This organic horizon is manifest regionally and
underlies most other lunettes on the Central Plateau including the Lake Augusta
lunette. Pharo (1990:13) refers to it as ‘coffee rock’ and ascribes it to the downward
leaching of organic material and iron oxides. On the basis of the date from Lake
Fergus, the Lake Augusta lunette probably is mid-Holocene in age.
Bradbury (1994) describes the Lake Augusta lunette. Its indented shoreline
comprises ‘Four Bays’ open to the prevailing westerly winds (Figure 2). These
shallow embayments are separated by elongate promontories with bouldery
headlands overlain by sand. Behind these embayments are parabolic dunes 5 m or
more in height forming the main body of the lunette. East of the lunette is a group
of smaller waterbodies known as the Carter Lakes. Some of these smaller
waterbodies have low-lying lunettes developed on their eastern margins.
Interestingly, these low lunettes are composed predominantly of quartzose sand
whereas the sand in the Lake Augusta lunette is doleritic. Bradbury suggests that
the reason is differential erosion of lakebed parent material.
Survey Methods
The geomorphology of the eastern side of Lake Augusta was mapped with the aid
of an enlarged (1:42,000) colour aerial photograph taken in 1997 (Plate 1). Major
landforms were traced from the photo and their exact boundaries and dimensions
established by close field inspection. The survey focussed on the main Lake
Augusta lunette and the landforms of the Carter Lakes area.
Subsurface sediments and soils were examined in dune blowouts, deflation
corridors and along the eroded foreshore of Lake Augusta. A hand auger was also
used where necessary. The sediments exposed were described with reference to soil
development, texture and stratification. This field evidence was used to reconstruct
a geomorphic history of the Lake Augusta lunette and subsequently to assess the
geoheritage significance of the lunette.
Significant areas of erosion were also noted. Enlarged black and white aerial
photographs taken of the lake in 1949 were examined to determine whether lake
level management since 1953 was likely to have contributed to the erosion of
landforms. Two profiles were constructed across the lunette and blowout surfaces
to determine whether lake level heights might be related to erosional processes.
The profiles were surveyed using a Trimble 4700 Real Time Kinematic GPS
Surveying System.
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Scientific Report on Lake Augusta
August 2003
Plate 1: Aerial photograph of Lake Augusta (17 Jan 1997, lake level at approx. 1147.3 m)
Geomorphic Characterisation
Nine geomorphic units were visually identified on the eastern shore of Lake
Augusta and around the Carter Lakes (Figure 2):
1. Sand lunette and lake shoreline ridges
2. Beach deposits
3. Rocky beach
4. Cuspate spit
5. Nearshore silt and clay
6. Low quartzose sand ridges
7. Low boulder plain
8. Deflation corridor
9. Eroded foreshore
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Scientific Report on Lake Augusta
August 2003
These units and their defining characteristics are described below. Particular
reference is made to any erosion observed in these units.
Sand Lunette and Lake Shoreline Ridges
The Lake Augusta lunette and associated lake shoreline ridges near the James River
outfall trend roughly south-west north-east. The main body of the lunette
comprises a series of coalescing parabolic dunes (Plate 1). The long axes of these
dunes are roughly aligned with the dominant wind direction and strike of the rocky
promontories forming the ‘Four Bays’. Individual parabolic dunes run eastward for
up to 250 m and rise up to 8 m above the lake. At the northern end of the lunette
behind One-Tree Beach the dune sand has advanced up the side of a low rocky hill
forming a climbing dune.
The sediments of the lunette and lake shoreline ridges are fine doleritic sand. A
weakly developed dune podsol appears to have developed in the main body of the
lunette whereas the lake shoreline ridges further to the north are reddish at depth.
These soil profiles are observable in deep blowouts caused by wind erosion and the
burrowing of wombats and rabbits. The lack of significant soil profile development
accords with the assumed late Holocene age of the landform.
The windward side of the lunette is particularly susceptible to erosion. Most
blowouts tend to occur on this side because of exposure to prevailing westerly
winds. Burrowing animals can exacerbate this wind erosion particularly on slopes
with a sunny northerly aspect. Reworking of the lunette by wind is evident in dune
blow-out sections. On the windward side of the lunette these show a buried
Holocene soil under about a metre of recently redeposited sand. The absence of
soil in this uppermost unit suggests that it may date from the European historic
period but because plant growth and soil formation is slow in cold climates it is
possible that this unit actually pre-dates the European period.
The leeward side of the lunette adjoining the Carter Lakes is relatively stable. The
noses of individual parabolic dunes comprise single or multiple slip faces at an
angle of repose close to 45 degrees. These have edged onto the floor of the Carter
Lakes and in some places present active slip faces. These active faces suggest some
ongoing advance of dune sand onto the floor of the Carter Lakes but in general the
sheltered side of the lunette appears well held by mature alpine vegetation.
Groundwater bevelling is a process that may be operating on lake margins that can
erode lake margins down to the level of the watertable. The watertable acts as a
base level to which deflation and weathering adjusts to produce a flat basin floor.
Evidence of erosion or sand movement that may be caused by groundwater
bevelling was suggested by the presence of ‘cliffing’ at the lake margin, although
this effect may also be caused by wave-cut during high water levels. Ice formation
and melting may cause further collapse of cliff-face sand and result in retreat of the
cliffline. Cliffline retreat results in expansion of the basin floor, the end result of
which is flattening of the land surface.
At Lake Augusta, all of these processes may be causing the shoreline to retreat,
although during the survey there appeared to be some evidence that groundwater
bevelling was active (Plate 2). In this instance, bevelling may be caused by a
perched watertable and sand movement is expressed by extensive low cliffing
along the lunette shoreline. As mentioned above, these cliffs may also have
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Scientific Report on Lake Augusta
August 2003
resulted from wave-cut, as well as groundwater pressure and frost heaving of wet
sand. This erosion begins as a sand slump approximately 1 metre across. Older
collapses are partially buried at the foot of the lunette and may also be impacted by
wombat burrowing. Data on the local changes in groundwater level, and their
relationship to surface water level changes in Lake Augusta, are needed to confirm
that this bevelling is an active processes causing these erosional features.
Plate 2: Fresh slump at the foot of the main lunette (to right of photo). This appears to be
undermining the lunette, causing retreat of the lunette cliff line.
Beach Deposits
Beaches are an integral part of sand lunette systems because they supply sand for
dune building. Beach deposits are present at Long Beach and Rocky Beach (see
Figure 2 for locations) and comprise sand and gravel derived mostly from the
underlying dolerite. The coarsest beach sediments have been worked by waveaction to the top of the beach forming low gravel berms. These berms are carpeted
by wrackline vegetation indicating the highest levels reached by the lake. At Long
Beach, a sequence of berms continues down the beach profile marking various lake
levels as water rises and falls.
The processes that are undermining the base of the lunette also appear to be
destabilising the beach deposits. The best example of this is a 1.5 m deep gully
formed at the southern end of Long Beach (Plate 3). This gully has cut a neat 1m
wide trench through Long Beach, and appears to channel groundwater flow and
surface run-off through to the lake. The result is a loss of sand from the beach.
This process is likely to be repeated at Rocky Beach where the watertable may be
similarly exposed above a small beach deposit.
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Scientific Report on Lake Augusta
August 2003
Plate 3: Gullying of beach deposits at southern end of Long Beach
Rocky Beach
Rocky beaches comprise outcrops of dolerite boulders. These protrude into the
lake forming highly localised concave shorelines. In some places isolated or lightly
clustered dolerite boulders stand in rows backed by beach deposits. Presumably
these boulders act as a barrier to wave attack (Plate 4). The rocky beaches are
favoured by pencil pines, which grow in small clumps above the high water mark.
Plate 4: Rocky Beach. Note groundwater seepage in background.
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Scientific Report on Lake Augusta
August 2003
Cuspate Spit
Cuspate spits are triangular deposits of beach sand which have their apex pointing
in the direction of the sea or lake from which they have emerged (Shepard, 1952).
They are formed when spits or beach ridges approach each other from opposite
directions owing to the action of two major wave sets or bi-directional currents.
The sharp projection of a cuspate spit seaward or lakeward may be maintained by
the presence of a nearby island, which protects it from direct frontal attack by
destructive waves.
A good example of a cuspate spit protected by a nearby island (or, in this case,
promontory) is located at the northern end of Short Beach (Plate 5). This deposit is
triangular in planform and consists of doleritic beach sand and gravel. It is about
50 m wide at its base and more than 0.5 m thick. This formation may have been
formed by reworking of beach sediments from One-Tree Beach and Short Beach.
The cuspate spit is located at the same height as the full supply level of the
modified Lake Augusta.
Plate 5: Cuspate spit looking east.
Nearshore Silt and Clay
This unit is best observed on aerial photographs. The silt and clay of the Lake
Augusta near shore zone represents the lower shore face of the lunette. A
comparable unit is absent from the Carter Lakes. The best surface expression of
this unit is available at the northern end of One-Tree Beach. An eroded section
shows yellow lacustrine silt and clay overlain by 30 cm of black organic soil and a
cover of sedge.
Low Quartzose Sand Ridges
These are low lunettes formed on the downwind side of four of the Carter Lakes.
The most easterly of these is the largest. It follows the outermost shoreline. The
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Scientific Report on Lake Augusta
August 2003
maximum height of this lunette was measured at 80 cm. At its widest it is about 40
m across. The other three low lunettes are probably no more than 60 cm high.
Unlike the main Lake Augusta lunette, these low lunettes are composed almost
entirely of fine quartzose sand. Some dolerite grains can be observed but these
represent only a small fraction of the total sediment population. These sediments
are aeolian in origin and seem relatively unweathered. The soil profiles of the low
lunettes are mostly undifferentiated and light brown in colour.
The Carter Lakes lunettes are in good condition. Wombat burrowing has disturbed
the most easterly lunette but the vegetation is quickly recovering. Groundwater
bevelling is a potential mechanism that may have caused the partial collapse of at
least one low lunette in this area (Plate 6).
Plate 6: Partial collapse of low lunette in Carter Lakes.
Low Boulder Plain
A low boulder plain composed of dolerite dominates the eastern side of Lake
Augusta and forms part of the lake floor when lake levels are raised. The Lake
Augusta lunette has been built over the low boulder plain. On the lee side of the
Lake Augusta lunette, the low boulder plain is well vegetated by thick alpine heath.
The elongate promontories dividing the foreshore of Lake Augusta are outliers of
the low boulder plain.
Deflation Corridor
Deflation corridors are erosion surfaces formed between the trailing arms of
elongate parabolic dunes formed by wind (Plate 7). Three significant deflation
corridors have formed in the Lake Augusta lunette and are likely to pre-date the
raising of Lake Augusta. The largest is approximately 200 m long and 60 m wide.
These deflation corridors are associated with the three longest parabolic dunes
formed at Long Beach and Rocky Beach. Deep blowouts (depressions formed by
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Scientific Report on Lake Augusta
August 2003
wind erosion) behind Short Beach are also beginning to form deflation corridors as
the parabolic dunes lengthen.
There is some visual evidence that the lower edges of the deflation corridor
erosion surfaces are at the level of the watertable. The deflation corridors also have
become small internal drainage basins redepositing sediment as fan deltas on the
lake margin. Erosion of the deflation corridors through a variety of mechanisms
(groundwater bevelling, sheet erosion, wave-cut, gullying, rainsplash, and wind
erosion) has exposed a humicrete layer (an ‘organic palaeosol’) beneath the lunette
and above the weathered dolerite of the underlying low boulder plain. The
deflation corridors are likely to have been present prior to Hydro Tasmania
development and therefore are unlikely to have been initiated as a result of raised
water levels.
Plate 7: Long deflation corridor behind Rocky Beach looking west. Note watertable exposed at
surface.
Eroded Foreshore
The eastern foreshore of Lake Augusta appears to be subject to erosion from
factors such as groundwater seepage, surface water movements and wind. The net
result appears to have been the removal of shoreface sediments and exposure of
the underlying weathered dolerite.
Erosion of the Lake Augusta foreshore appears to progressively worsen in the
direction of the James River outfall. At the southern end of the Lake Augusta
lunette, Long Beach is in relatively good condition presenting a steep beach profile
and an abundance of beach sediment. Northward, however, beach sediments
gradually diminish in their extent until almost absent from Short Beach and OneTree Beach. These last two beaches are likely to have been reworked to form the
cuspate spit at the northern end of Short Beach. The timeframe over which this
loss of sand has occurred is unknown, as are the processes that have been the main
factors causing this to take place.
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Scientific Report on Lake Augusta
August 2003
Some of the worst foreshore erosion can be seen at One-Tree Beach (Plate 8).
Below the high water mark are small sand islands and pedestals of vegetation
representing remnants of the original shoreline and lower shoreface. Stripping of
the shoreface has also exposed the humicrete layer and underlying dolerite. The
original beach has been reduced to a thin sand mantle. The extent of shoreline
retreat at this location can be gauged by a lone large shrub (the One-Tree) clinging
by its exposed roots to the edge of this sand mantle (Plate 9). This suggests the
lunette shoreline may have retreated by at least 10m.
Plate 8: Eroded foreshore along One-Tree Beach. Beach sediments have been removed by wave
action and currents exposing underlying weathered dolerite.
Plate 9: The “One Tree” on One-Tree Beach. Note shoreline retreat of about 10m.
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Scientific Report on Lake Augusta
August 2003
Summary
Erosion at Lake Augusta appears to stem from six main processes. These are:
!
lengthening of some parabolic dunes and maintenance of deflation corridors;
!
the action of waves and currents during high lake levels;
!
groundwater bevelling;
!
aerial processes causing lunette blowouts;
!
subaerial processes such as the action of frost and ice; and
!
wombats and other burrowing animals such as rabbits.
Active management of water level in Lake Augusta has the potential to affect
surface water processes for short periods of time and groundwater for longer
periods. While wave action during artificially high water levels clearly has the
capacity to initiate and progress erosion on the windward side of the lunette, there
is also some potential that groundwater bevelling may contribute to this. It is
possible that groundwater discharge associated with a perched water table in
Carters Lakes may be the cause, although detailed hydrological modelling is
required to confirm this hypothesis.
Post-1953 Erosion Processes
Comparison of aerial photographs taken of Lake Augusta in 1949 with present-day
aerial photographs shows that construction of the dam in 1953 and subsequent
lake level management did not initiate the large dune blowouts and long deflation
corridors in the large sand lunette. Furthermore, analysis of the vegetation cover
shown on these photos indicates that the size of these wind-eroded areas has not
changed greatly in the past 50 years. This interpretation accords well with the
observation that the leeward side of the lunette is relatively stable; although active
slip faces in some areas suggest some ongoing advance of the dune sand.
Wind erosion may not be as significant as other processes operating on the lunette
today. One reason may be that when the strongest westerly winds blow over the
winter and spring months the sand is moist or waterlogged and difficult to entrain.
Nonetheless, wind erosion is ongoing as indicated by the numerous blowouts of
varying size on the windward side of the lunette. These blowouts appear to have
been initiated by rabbits or possibly wombats. One possible trigger for the
mobilisation of the long parabolic dunes was the rabbit plague of 1910. However,
the formation of elongate parabolic dunes is a normal part of sand lunette
construction (Lees, 1989; Lees and Cook, 1991).
Field observations made during this survey suggest that water and subaerial
processes are likely to be causing erosion along the waterline. Lake level
management since 1953 may have contributed to this erosion by causing levels in
Lake Augusta to be increased. It is possible that such increases have resulted in
destabilisation of the shoreline. Possible evidence of destabilisation by surface
water is present at One-Tree Beach and Short Beach where beach sediments
appear to have been stripped and reworked into a high-level cuspate spit. While the
fact that the cuspate spit lies at about the full supply level supports the conclusion
that this was formed following the construction of Augusta Dam, this
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Scientific Report on Lake Augusta
August 2003
interpretation cannot be supported by the pre-dam photo because of the poor
quality of the enlargement.
Further evidence that water-related processes are resulting in erosion is the low
cliffing on the windward side of the main lunette. This occurs about 3 m above the
level that Lake Augusta was at during the field survey (between 1147.6 m and
1147.8 m), which coincides with full supply level (1150.6 m above seal level) as well
as being at the same level as the Carter Lakes. The cliffing which occurs at this
level on the lunette may therefore be caused by wave-cut during water levels
associated with full supply, or by groundwater bevelling, when water from the
Carter Lakes flows through the lunette and discharges above the water level in
Lake Augusta following wet periods.
It has been hypothesised that the Carter Lakes are a surface exposure of the local
watertable or ‘groundwater window’. The low cliffing on the windward side of the
lunette at the level of the watertable could be taken as evidence that groundwater
discharge may be the cause. This process is more commonly observed in the playa
lakes of the Murray Basin where it is known as groundwater bevelling (Bowler,
1986). The watertable acts as a base level to which deflation and weathering adjusts
to produce a flat basin floor. It also causes lake basins to expand laterally. A
graphical representation of this process for the Lake Augusta situation is presented
in Figure 3. Groundwater bevelling of playa lakes is exacerbated by salt weathering
in the zone above the watertable resulting in cliffing (Bowler, 1986). A similar
effect may be caused at Lake Augusta by the action of frost and ice in the capillary
fringe. This subaerial process has been described by Lawler (1989). When water in
the capillary zone freezes, plant roots are attacked by needle ice causing the death
of the plants and subsequent slumping of the soil. Prosser et al, (2000) observed
that this process was most intense near areas of groundwater seepage. At Lake
Augusta, the combined effect of groundwater seepage and heaving by frost and ice
may be a factor causing lunette scarping.
Lake level management since 1953 may have contributed to this process. By raising
lake levels, it could be assumed that the watertable and capillary fringe were also
raised. Under original conditions, the watertable was likely to have been below the
humicrete layer (located at 1150.9 metres above sea level). Assuming this layer is
continuous and impermeable, this may have acted as a confining unit for local
groundwater. Under the new water level regime, the watertable may now be
perched above the humicrete layer in unstable aeolian sand. This perched
watertable would be recharged by high lake level events, and when lake levels fall
again the pressure of groundwater discharge and the effects of frost and ice may
cause bevelling of the shoreline. Under this scenario, water levels in the Carter
Lakes remain high after the levels of Lake Augusta fall because they are closed
lakes maintained by local rainfall and run-off. It must be pointed out that this is a
hypothesis only and that additional data on groundwater level changes in the area
are needed to confirm or refine this idea.
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Scientific Report on Lake Augusta
A)
August 2003
Large
sand
lunette
‘Normal’ lake
levels
Lake Augusta
Carter Lakes
Humicrete
Lake level
Lake level
FSL
Watertable
B)
Raised lake
levels at FSL
Lake level
C)
Humicrete
FSL
Lake Augusta levels fall through James River outfall following flood event. Carter Lakes
remain high because they are closed lakes and actually fill further from rainfall and local
run-off.
Perched watertable
Lake level
D)
FSL
Groundwater pressure or ‘seepage force’ causes cliffing at the lunette shoreline or
‘groundwater bevelling’
Lake level
Flow
FSL
Figure 3: Possible influence of raised levels in Lake Augusta on the water table
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Scientific Report on Lake Augusta
August 2003
Assessment of Geoheritage Significance
Geoheritage is recognised by the Australian Heritage Commission Act 1975 as any
‘site’, ‘feature’, ‘geological monument’, ‘place’ or ‘area’ that has ‘aesthetic, historic,
scientific or social significance or other special value for future generations as well
as for the present community’. Criteria for assessing geoheritage significance in
Australia were proposed by the Standing Committee for Geological Heritage of the
Geological Society of Australia (Joyce, 1995). The term ‘feature’ best covers items
of geological or geomorphological interest. Such features may be:
‘an outcrop or natural section, a man-made exposure such as cutting, quarry or
mine, an unconformity or fault-plane, or a landform, landscape or viewpoint. It
may be a continuing geomorphological process such as coastal cliff erosion by
waves’ (Joyce, 1995:15).
Significant geomorphological features may be listed on the Register of the National
Estate. The Australian Heritage Commission requires that the RNE ultimately
contain:
1. a representative list of the places which demonstrate the main stages and
processes of Australia’s geological history; and
2. rare or outstanding natural phenomena, formations, features, including
landscapes and seascapes (Joyce, 1995:16).
The level at which a feature is given significance may range from the local to
international level. Assessment is based on the importance of the site in terms of
similar sites elsewhere known to the person making the assessment (Joyce,
1995:A1.5).
Significance of Lake Augusta
Lake Augusta is likely to be significant at a regional level because it is one of only
seven lakes on the Central Plateau known to have developed a sand lunette. This
lunette is an example of sand lunette formation in a high alpine environment. Sand
lunettes of this type are not known in any other part of the world.
Without dating of the Lake Augusta lunette and associated shoreline ridges, it is
difficult to assess which particular stage of Australia’s geological history these
features represent. However, it is unlikely that the Lake Augusta lunette formed
during a ‘Mid-Holocene Arid Period’ so the lunette has little palaeoclimatic
significance. The possible aspect of Lake Augusta and its lunettes that may be
significant is that they present an unusual example of Lees’ (1989) and Lees and
Cook’s (1991) lake segmentation theory and its applicability to lunette formation.
Furthermore, the Lake Augusta sand lunette formed over a rocky substrate rather
than by shoaling which is a rare natural phenomenon not previously observed. For
these reasons, Lake Augusta and its lunettes may be considered as significant,
however the Lake Augusta lunette environment is not currently listed on the
Register of the National Estate.
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Scientific Report on Lake Augusta
August 2003
Native Fish Surveys
Population status of P. julianus
Surveys of native fish populations were conducted using backpack electrofishing
equipment, sampling a variety of habitats around the margins of the natural lake
and impounded areas of Lake Augusta. These surveys confirmed healthy galaxiid
populations, with Paragalaxias julianus, Galaxias brevipinnis and Galaxias truttaceus
captured in significant numbers and a range of size classes around the margins of
the natural lake. Few galaxiids, however, were captured in the James River
upstream of Lake Augusta during these surveys.
The surveys found that Paragalaxias julianus, Galaxias brevipinnis and Galaxias
truttaceus were present in the Augusta impoundment and the Ouse River upstream
of the impoundment, but all three species were captured in much lower numbers
in these regions of the lake in comparison to the natural lake.
Habitat preference data for P. julianus
The electrofishing surveys conducted in the natural lake and impoundment
provided a general indication of P. julianus habitat preferences. The rocky margins
of the natural lake, particularly those located around its western half, appeared to
be preferential habitat for P. julianus. Large numbers of fish were collected from in,
around and from underneath the cobbles and boulders in these shoreline areas.
Macrophyte beds situated in the western half of the natural lake were also
electrofished, however few paragalaxiids were observed or collected from these
areas. No paragalaxiids were captured from the open sandy habitats around the
lakes eastern shores.
The handful of galaxiids collected from the Augusta Impoundment also appeared
to be associated with rocky marginal areas, with few fish captured from the open
areas of silt that cover a significant proportion the lake bed, particularly at low
water levels. The benthic habitat of the impoundment differs markedly to that of
the natural lake. The majority of the rocky habitat around the impoundment
consists of boulders that are imbedded into the substrate, with little interstitial
refuge habitat available due to extensive siltation in the bed of the impoundment.
Formation of the Augusta Impoundment has modified the hydrology of the
natural lake. When the impoundment fills, it backfloods into the natural lake,
however the original lake essentially maintains its natural extent during the drier
months and is elevated during spill events and high lake levels in the
impoundment. The effects of the slightly modified water level regime in the
natural lake on native fish populations is not clear, however large numbers of
paragalaxiids were captured during the recent surveys, indicating that the existing
water level regime in the natural lake is suitable for the maintenance of paragalaxiid
populations. The Augusta Impoundment appears to marginal habitat due to large
seasonal variations in benthic habitat availability, much of which appears suboptimal due to heavy siltation.
Recreational Trout Fishery
Data collected by the Inland Fisheries Service from angler catch returns was
collated in order to determine whether any relationship existed between lake level,
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Scientific Report on Lake Augusta
August 2003
angler visitation levels and catch rates, however there was insufficient data to allow
meaningful analysis of this information. It should be noted that Lake Augusta and
Lake Mackenzie are the only western lakes where bait fishing is permitted.
Analysis of aerial photos combined with visits to the lake have given indications
that the aquatic macrophyte beds located in the western half of the natural lake
may be decreasing in size. It is not clear whether the apparent decline is associated
with normal inter-annual variability, or whether there is a long-term decrease in
macrophyte cover within the lake. The relationship between recreational fishery
productivity and macrophyte extent is also unclear.
4. ENVIRONMENTAL MANAGEMENT OPTIONS FOR LAKE
AUGUSTA
Geomorphology
Through a process of visual interpretation, the geomorphic assessment identified
and characterised 9 units in and around the Lake Augusta and Carter Lakes area.
The assessment also identified those units that may be susceptible to erosion
processes and posed some hypotheses regarding hydrologic mechanisms that may
be influencing erosion. Unfortunately, the lack of detailed information on water
movement meant that some of these hypotheses are unsubstantiated, and further
work is required to resolve this situation. It is likely that the Lake Augusta lunettes
have some geoheritage significance, at least at the regional level.
Regarding the erosion of landforms, it is likely that the artificially high lake level
events cause wave-cut scarping on the windward side of the lunettes between Lake
Augusta and the Carter Lakes, although there is an alternative hypothesis that this
scarping has been produced by groundwater bevelling due to an artificially high
watertable perched above the normally confining humicrete layer. Whether the
watertable is artificially or naturally perched, it may be regularly recharged by high
lake levels in Lake Augusta. Further data to resolve this situation is required.
Surface water erosion may also be responsible for changes at the northern end of
the main lunette where two beaches may have been stripped and reworked by wave
action into a high-level cuspate spit. This is less of an issue than the erosion
occurring to the windward side of the lunette.
Another apparently significant issue may be the dune blowouts, particularly those
exacerbated by rabbit and wombat burrowing. Remedial attention may be required
to treat these blowouts, but this has not been identified as an issue arising from
water level management.
A significant issue that was raised during the study was the mechanisms by which
scarping is being caused at the base of the lunette. Further study and some
monitoring is required to establish whether this is being caused by wave-cut or
groundwater bevelling, as any remedial activities that are undertaken must target
the cause and not the symptom.
In light of this, the following options for management might be considered:
!
Establish groundwater monitoring devices to resolve the question regarding
groundwater bevelling.
South Esk - Great Lake Water Management Review
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Scientific Report on Lake Augusta
August 2003
!
Develop a foreshore protection strategy to address the issue of dimunition of
sand for dune-building resulting from beach erosion. Sand replenishment, or
sandbagging to reduce scarping may be worth considering, depending on the
threatening process.
!
Consultation with the Tasmanian Aboriginal Land Council to establish the
significance of Aboriginal stone artefacts present in the dune blowouts and
along the eroded foreshore, and develop appropriate measures to protect
them if necessary.
Threatened Fish
Hydro Tasmania operations in Lake Augusta do not appear to be having a
detrimental effect on populations of P. julianus in the natural impoundment, and so
specific management prescriptions for the species have not been recommended.
While the abundance of P. julianus appears relatively low in the Augusta
Impoundment, initial assessments have indicated that the impoundment provides
relatively poor habitat and is located on the outer extreme of the species
distribution. At this stage no native fish management recommendations have been
proposed for the impoundment. Hydro Tasmania will continue to support the
recovery plan for the species through participation on the Tasmanian threatened
galaxiid recovery team.
Recreational Fishery
Options to address recreational fishery issues include:
!
!
Provide accurate public information on when the lake is spilling or about to
spill.
Manage maintenance activities associated with Augusta Dam so that impacts
on angler access and the trout population are minimised where possible.
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Scientific Report on Lake Augusta
August 2003
References
Banks, M. 1973, The lake country of Tasmania, Symposium conducted by the Royal Society
of Tasmania, Foot and Playsted, Launceston.
Bowler, J. 1986, “Spatial variability and hydrologic evolution of Australian lake basins:
analogue for Pleistocene hydrologic change and evaporite formation”.
Palaeogeography, Palaeoclimatology, Palaeoecology, 54:21-41.
Bradbury, J. 1994, Aeolian landforms in the Lake Ada – Lake Augusta area: a preliminary
investigation and management strategy. Unpublished draft report. Parks and Wildlife
Service, Hobart.
Cosgrove, R. 1984, Aboriginal Economy and Settlement in the Tasmanian Central Highlands,
Parks and Wildlife Service, Hobart.
Cullen, P. 1995, “Land degradation on the Central Plateau, Tasmania: the legacy of 170
years of exploitation”, Occasional Paper No.34, Parks and Wildlife Service, Hobart.
Crawley, M. J. 1986, Plant Ecology, Blackwell Science, Oxford.
Du Cros, H. 1992, An Archaeological Survey to Examine the Nature and Distribution of
Aboriginal Sites in Areas Affected by Erosion. Parks and Wildlife Service, Hobart.
Environmental Services 1999, Environmental Review: South Esk - Great Lake Hydro
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